In the past decades, lithium-ion batteries (LIBs) have been widely utilized in various applications especially consumer electronics because of their superior energy density, long cycle life and discharging capability. Due to rapid market development of electric vehicles (EV) and grid energy storage, high-performance, low-cost LIBs are currently offering one of the most promising options for large-scale energy storage devices.
The use of multi-element lithium transition metal oxide such as lithium nickel manganese cobalt oxide (NMC) and lithium nickel cobalt aluminum oxide (NCA) has become popular due their superior electrochemical properties over traditional cathode material such as LiMnO2, LiCoO2, and LiNiO2. High nickel cathode material exhibits a high energy density and superior capacity property.
Currently, a cathode slurry may be prepared by dispersing a cathode active material, binder material and conductive agent in an organic solvent such as N-methyl-2-pyrrolidone (NMP). The cathode slurry is then coated onto a current collector and dried to prepare a cathode.
The use of aqueous solutions instead of organic solvents is preferred for environmental and handling reasons and therefore water-based slurries have been considered. However, high nickel cathode material is sensitive to water exposure. Upon exposure to water, lithium at the surface of the cathode material reacts with water thereby resulting in the formation of soluble bases. The high content of soluble base will affect the pH of the cathode slurry. However, pH values outside certain ranges (e.g., above pH 10) may affect dispersion homogeneity of components (e.g., cathode active material) in the cathode slurry and binding strength of the binder material, and can also have negative effects on the metallic components of the electrode (e.g., the metallic collector). These factors all contribute to poor electrochemical performance. Conventionally, a pH modifier is used to adjust the pH of the cathode slurry. However, additives may also have a deleterious effect on the electrochemical processes that take place at the cathode, especially at higher voltages and high temperature, which in turn reduces battery performance. Accordingly, it is desirable to adjust a pH of the cathode slurry without adding any additives.
CN Patent Application No. 105762353 A discloses a method of preparing a lithium-ion battery having high nickel ternary cathode material. The method comprises mixing a high nickel ternary cathode material with a conductive agent in a blender to obtain a mixture; adding a binder and water to the mixture with mixing; adding more water to the mixture until reaching a certain viscosity. However, the cycle life of the batteries prepared by this method is less than 360 cycles in terms of 20% loss of its initial capacity, which is insufficient for many intended applications such as portable electronics and electric vehicles.
CN Patent Application No. 105261753 A discloses an aqueous cathode slurry and a preparation method thereof. The aqueous cathode slurry comprises a cathode active material (25%-35%), a carbon nanotube (12%-20%), a conductive agent (6%-10%), an aqueous binder (4%-6%) and water (40%-50%). The method comprises mixing a binder with water to obtain a pre-mixed solution; adding a carbon nanotube and a conductive agent to the pre-mixed solution to obtain a conductive gel solution; milling the conductive gel solution until the milled material has a fineness from 5 μm to 10 μm; adding a cathode active material and more water to the milled conductive gel solution with mixing; vacuum pumping the slurry; leaving the slurry to stand for a while to obtain an aqueous cathode slurry. However, there is no data to evaluate the electrochemical performance of a battery using ternary transition metal oxide as a cathode active material.
In view of the above, there is always a need to develop a method for preparing cathode slurries having high nickel cathode material for lithium-ion batteries with good electrochemical performance using a simple, fast and environmentally friendly method.